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Hypothetical Cell System
Operation
In Figure 1: "Hypothetical Cell System", the area
bounded by bold lines represents the total
coverage area of a cellular system. This area is
divided into several cells, each containing a cell
site base station which interfaces radiotelephone
subscribers to the switching system. Since there
are no reserved channels for each cell in CDMA,
a user has a better chance of completing a call.
Also, now there is no hard handoff between cell
sites since all sites operate on the same
frequency. This is called soft handoffs. In this
system, subscribers in cell A & D simul-
taneously operate in the same frequency. As a
user moves from cell site to cell site, the base
station monitors the signal strength of the user.
Based on this signal strength, the base station
decides which cell shall carry the call.
When a radiotelephone is in use well within a
cell, the signal strength received at the cell site
base station will be high. As the phone is moved
towards the edge of the cell, its received signal
strength decreases. Signal strength information
therefore
provides
an
subscriber's distance from a cell's base station.
This change is handled automatically, and is
completely transparent to the user. For example,
assume that a cellular tele-phone initiates a call
in cell A and then moves across the system area
through cells B and C to cell D. As the phone
moves into cell B, it is instructed to change to a
different frequency that operates through the B
cell on that frequency. A similar change is
performed when the phone moves from cell B to
cell C and again when the phone moves from cell
C to cell D.
In this example, the radiotelephone has operated
in four cell sites, through four cell sites, and on
the same spread spectrum without interruptions
in voice communications. As the radiotelephone
leaves a cell, the frequency on which the phone
and base station were operating is made available
to another subscriber in that cell. Since this
radiotelephone is dual mode, the radiotelephone
can operate in either a CDMA system or Analog
system.
indication
of
the
16
Cellular Overview
Service Area
The area within which calls can be placed and
received is defined by the system oper-ator.
(Because this is a radio system, there is no exact
boundary that can be drawn on a map.) If the
portable is outside the radio service area, a No
Svc (no service) message will appear on the
phone's display, and calls cannot be placed or
received. If this happens during a conversation,
the call is lost. Places where the ability to place
or receive calls would be lost are in totally
enclosed areas, such as underground parking
garages, in buildings without windows, and in
elevators. This situation would be indicated
either by the No Svc message illuminating, or by
the sound of either a fast busy signal or a high-
low siren signal when call placement is
attempted.
General usage in buildings having reason-able
glass area is usually quite good. However, it may
be necessary to move closer to a window to
ensure reliable opera-tion.
PCS System
(Personal communication System) is identical to
this cellular system except that the radio tele-
phone service in the frequency range of 1850
MHZ to 1990 MHZ and the duplex spacing is 80
MHZ.
1/2/02
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